Process of producing radio-active elements



June 13, 1939. L. SZILARD 2,161,985

PROCESS OF PRODUCING RADIO-ACTIVE ELEMENTS Filed March 11, 1935 3 Sheets-Sheet 1 Jun 13, 1939. sz 2,161,985

PROCESS OF PRODUCING RADIO-ACTIVE ELEMENTS June 13, 1939. sz 2,161,985

PROCESS OF PRODUCING RADIO-ACTIVE ELEMENTS Filed March 11, 1935 3 Sheets-Sheet I5 Patented June 13, 1939 NKTED ST S PATEN FFHC Z1251 PROCESS OF PRODUCING RADIO-ACTIVE ELEMENTS Leo Szilard, New York, N. Y.

9 Claims.

This invention concerns methods and apparatus for the generation of radio-active bodies.

According to one feature of my invention, radio-active elements may be produced from nat- 5 ural elements by bombarding a natural element or compounds of natural elements with neutrons produced in various ways, more particularly, by subjecting the natural elements to neutrons emanating from a target containing lithium, which 10 target is subjected to a bombardment with last deuterons. Another feature of the invention is directed to the production of radio-active elements from natural elements by exposing the natural elements to an irradiation with neutrons I5 which are liberated from certain elements under the action of X-rays. Another feature of the invention is directed to chemically concentrating radio-active elements produced from natural elements if the radio-active element is isotopic a with the natural element from which it is produced.

Other features of the invention will appear in the following detailed description referring to the drawings, and will be more particularly pointed 25 out in the claims.

In the drawings,

Figure 1 represents a sectional elevation of an apparatus for carrying out the invention, 2

Figure 2 shows a more constructional lay-out 39 of the apparatus of Figure 1,

Figure 3 shows the circuit arrangements for further modified apparatus and,

Figure 4 is a sectional view of apparatus intended to co-operate with that shown in Figure 3.

Referring first to Figure 1 of the drawings, l I

is an electrical discharge tube adapted to project a beam l2 of fast deuterons. The tube II is filled with deuterium and an anode A and cathode B are provided for connection to a source 40 of high voltage. The deuterons are thus pro- J'ected at high speed and pass through the cathode B. The deuterons fall on a substance IS in a sealed container BA. The substance l3 consists, for instance, of lithium. The collision of the fast diplogen ions with the substance l3 causes transmutation, i. e. a nuclear reaction of the deuteron with an atom of the target. The substance I3 is surrounded by a thick layer l4 containing the element which it is desired to transmute into a 5 radio-active element. In order to have a high efliciency, the thickness of the layer I 4 has to be sufiiciently great, compared with the mean free path of the neutron, to prevent escape of any of the neutrons.

55 Figure 2 shows in more detail the electrical discharge tube H referred to in Figure 1. The tube essentially consists of a main portion I 6 serving to accelerate the deuterons and an auxiliary tube T for initiating the flow. A is the anode and IS the cathode of the auxiliary tube, deuterium being admitted thereto through the inlet 13B and being pumped away through the outlet MA. The flow initiated by the auxiliary tube is accelerated by passage through the main tube It which is maintained exhausted by suction outlets I 4 and I4, and which has a high potential gradient, there being a million volt potential difference between the ends of the tube. The accelerated deuterons emerge through the neck I4 of the tube l6 and collide with the substance l3 as de- 15 scribed with reference to Figure 1 of the drawmgs.

If the substance I3 is a light element for instance lithium, then the bombardment by the accelerated deuterons results in emission of uncharged particles of mass of the order of magnitude of the mass of a proton. Such uncharged nuclei 1. e. neutrons, penetrate even substances containing the heavier elements without ionisation losses, and will cause the formation of radioactive substances in the layer I4 exposed to them.

It is to be noted that by the method so far described, the ionisation losses suffered by the deuterium nuclei are comparatively small in light elements and also that the substance to be made radio-active is irradiated with neutrons i. e. uncharged nuclei, which pass through even heavy elements without ionising them. The substance l4 exposed for treatment by the neutron radiation may be in the form of an organic compound for the purpose of carrying out separation of the generated radio-active element, as described more fully hereinafter.

Neutron radiation may also be produced by the action of X-rays upon an element having a dissociable neutron at the prevailing voltage, and apparatus for carrying out this process will now be described with reference to Figure 3 of the drawings.

In Figure 3, I is the primary of a transformer, the secondary 2 of which is connected to the junctions 3 and 4. The junction 3 is connected to the cathode 8 of the rectifier tube 5 and to the anode I of the rectifier tube 6. The junction 4 is connected to the cathode 9 of the rectifier tube l0 and to the anode ll of the rectifier tube l2. The cathodes l3 and I4 are connected to each other and to earth. The anodes l5 and I6 are connected at H, and fom this point are connected to the pole I of the impulse generator 20, 56

the pole 19 of which is connected to earth. The impulse generator 20 is built of condensers 2|, resistances 22 and spark-gap devices 23.

The impulse generator and rectifying unit shortly described above, are known components adapted to give an extremely high voltage for a fraction of a second. With such a system voltages up to 3 million volts have been obtained. The negative side of the impulse generator is connected to a spark gap device 25, which in turn is connected with the cathode 26 of the discharge tube 24. The latter is built up from rings 24A of which only a few are shown-in the drawings. It will, however, be understood that the rings are continuous to enclose a space which is exhausted through the outlet 24B. The anode 21 of the tube is connected to earth and is formed by a metallic window. A body of material 28 is arranged at the external side of the window 21.

.20 When the impulse generator operates to produce discharge between the cathode 26 and anode 27 of the tube 24, fast electrons penetrate the anode 21 and impinge .upon the body 28. The latter when formed of Bi or Pb or some other heavy element, efliciently acts as an anti-cathode and hard X-rays are produced.

In Figure 4 of the drawings there is shown the lower portion of the discharge tube 24 with a device therebeneath for utilising the hard X-rays capable of being produced with the aid of the fast electrons emerging through the anode 21 of the tube 24. The device consists of a block 34 of the element which is to be made radioactive, a block 32 of an element with a dissociable neutron, being located therein. An aperture is formed in both the blocks 32 and 34 to allow entry of the cathode rays from the tube 24 above. The blocks 32 and 34 are also arranged to accommodate a wheel 30 and axle 35. The wheel 30 at its periphery carries a covering of tungsten or lead 3|. The covering 3| acts as an anticathode and is cooled with water introduced along the bearing for the axle 35. The block 34 may be in the form of a cube having a length of side of 50 cm., whilst the block 32 can also be of cube form with a side of 25 cm. For the sake of example the block 34 may be formed of iodine or arsenic or other material which lends itself to being made radio-active. The block 32 may be of metallic beryllium. In order that an isotopic separation as described hereinafter may be performed after irradiation the material of the block 34 may be in the form of an organic compound. A voltage of 3 million volts may be used for the discharge tube and inoperation the wheel 30 is rotated so that electrons passing through the anode 21 of the tube 24 hit the rotating anticathode covering 3!. When the fast electrons strike the anti-cathode, hard X-rays are pro duced which penetrate the beryllium block 32 and cause neutrons to be released therefrom, which neutrons then act upon the block 34.

It may be that fast electrons and hard X-rays have a similar eifect upon beryllium and one may therefore contemplate the making of the covering 3| of the wheel 30 from beryllium, the beryllium block 32 then being dispensed with, so that the neutrons released directly from the beryllium anti-cathode may enter and act upon the block 34.

It is found that when various elements are irradiated with neutrons by the process described above, practically all elements which become radio-active transmute into their own radioactive isotopes, and it becomes diflicult to separate these radio-active isotopes from the remaining portion of the element unaffected. In order to achieve separation of the radio-active element from the non-radio-active part thereof the following process may be adopted. This process is based on the fact that if a compound of an element is irradiated by neutrons, and if an atom of the element transmutes into the radio-active isotope. then this atom is freed from the compound. In accordance with the process, a compound of the element it is desired to make radioactive is chosen such that the freed radio-active isotope of the element will not interchange with the combined atoms of the element within the compound, whereby the freed isotope may be chemically separated from the irradiated compound. Very often the element whose radioactive isotope is to be isolated, can be conveniently irradiated in the form of a compound in which it is bound to carbon. Thus in the case of iodine compounds such as iodoform or ethyl iodide, the radio-active iodine isotope may be chemically separated from the original iodine compound in the form of free iodine. In order to protect the radio-active iodine isotope a small amount of normal iodine may be dissolved in the organic iodine compound before irradiation or after irradiation but before separation.

What I claim and desire to secure by Letters Patent of the United States is:

1. The method of producing a radio-active element from a natural element by causing fast deuterons to impinge on a target containing lithium, and exposing a layer of the natural element to be transformed into a radio-active element to the neutron radiation emitted by the said target.

2. The method of producing from a natural element a concentrate of a radio-active element which is isotopic with the said natural element, which comprises subjecting a compound of said natural element to. an irradiation which will transform some of said natural element into a radio-active isotope of said natural element, said compound of said natural element being one which in the environment in which the irradiation is being carried out does not interchange atoms of said natural element bound in the compound with atoms of said natural element or its isotopes outside the compound, and separating, after irradiation, from the compound said natural element and its isotopes which are outside the compound.

3. The method of producing from a natural element a concentrate of a radio-active element which is isotopic with said natural element, which comprises subjecting a compound of said natural element to irradiation with neutrons which will transform some of said natural element into a radio-active isotope of said natural element, said compound of said natural element being one which does not interchange in the environment in which the irradiation is carried out, atoms of said natural element bound in the compound with atoms of said natural element or its isotopes outside the compound, and separating, after irradiation, from the compound said natural element and its isotopes which are outside the compound.

4. The method of producing from a natural element a concentrate of a radio-active element which is isotopic with said natural element, which comprises irradiating with neutrons an organic compound of said natural element which will not interchange atoms of said natural element bound in the compound with atoms of said natural element or its isotopes outside the compound, and separating, after irradiation, from the compound said natural element and its isotopes outside the compound.

5. The method of producing from a natural element a concentrate of a radio-active element which is isotopic with said natural element, which comprises irradiating with neutrons a compound which contains carbon, in which said natural element is bound to carbon and which compound will not interchange atoms of said natural element bound in the compound with atoms of said natural element or its isotopes outside the compound, and separating, after irradiation, from the compound said natural element and its isotopes outside the compound.

6. The method of producing from a natural element a radio-active element which is isotopic with the natural element comprising the steps of producing fast electrons, directing them toward a target adapted to produce X-rays under the impact of said electrons, exposing to the action of said X-rays an element of the class consisting of beryllium and heavy hydrogen which produce neutron radiation under the action of said X-rays, and producing a radio-active element from a natural element by exposing the natural element to said neutron radiation.

7. The method of producing from a natural element a radio-active element which is isotopic with the natural element comprising the steps of producing fast electrons having an energy of at least 3,000,000 volts, directing them toward a target adapted to produce X-rays under the impact of said electrons, exposing to the action of said X-rays an element of the class consisting of beryllium and heavy hydrogen from which neutrons are liberated by X-rays of 3,000,000 volts energy, and producing a radio-active element from a natural element by exposing the natural element to said neutron radiation.

8. The method of producing from a natural element a radio-active element which is isotopic with the natural element comprising the steps of producing fast electrons, directing them toward a target adapted to produce X-rays under the impact of said electrons, exposing beryllium to the action of said X-rays to produce neutron radiation, and producing a radio-active element from a natural element by exposing the natural element to said neutron radiation.

9. The method of producing from a natural element a radio-active element which is isotopic with said natural element comprising the steps of producing fast electrons, directing them toward a target adapted to produce X-rays under the impact of said electrons, exposing to the action of said X-rays an element of the class consisting of beryllium and heavy hydrogen which produce neutron radiation under the action of said X-rays, and irradiating by said neutron radiation a compound of said natural element which in the environment in which said irradia tion is carried out will not interchange atoms of said natural element bound in the compound with atoms of said natural element or its isotopes outside the compound, and separating, after irradiation from the compound said natural element and its isotopes outside the compound.

LEO SZILARD. 

